Catalytic conversion of methanol to light olefins

ABSTRACT

A process for converting a methanol feed to light olefins over a zeolite catalyst comprising at least some crystalline aluminosilicate zeolitic material having a Constraint Index of about 1 to 12 and a silica/alumina mole ratio of at least about 12, e.g., ZSM-5, in the presence of an aldehyde-containing diluent comprising formaldehyde, acetaldehyde or mixtures thereof, under methanol conversion conditions. By using such a ZSM-5 type zeolite catalyst and an aldehyde-containing diluent, methanol can be converted to an olefin-containing hydrocarbon product enriched in C 2  to C 4  olefins such as ethylene.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for converting a methanolfeed to light olefins over crystalline aluminosilicate zeolitecatalysts.

2. Description of the Prior Art

A remarkable growth in the production of synthetic fibers, plastics andrubber has taken place in recent decades. Such growth, to a largeextent, has been supported and encouraged by an expanding supply ofinexpensive petroleum raw materials such as ethylene and propylene.However, increasing demand for these light olefins has, from time totime, led to periods of shortage, either due to a diminished supply ofsuitable feedstocks or to limited processing capacity. In any event, itis now considered highly desirable to provide efficient means forconverting raw materials other than petroleum to light olefins.

One such non-petroleum source of light olefins is coal-derived methanol.In this respect, it is known that methanol can be catalyticallyconverted to olefin-containing hydrocarbon mixtures by contact undercertain conditions with particular types of crystalline zeolite catalystmaterials. U.S. Pat. No. 4,025,575, issued May 24, 1977, to Chang et al.and U.S. Pat. No. 4,083,889, issued Apr. 11, 1978 to Caesar et al., forexample, both disclose processes whereby methanol and/or methyl ethercan be converted to an olefin-containing product over a ZSM-5 type(Constraint Index 1-12) zeolite catalyst. ZSM-5, in fact, convertsmethanol and/or methyl ether to hydrocarbons containing a relativelyhigh concentration of light (C₂ and C₃) olefins.

It is known that such light olefin production from the catalyticconversion of methanol can be optimized by varying one or more reactionparameters. Modification of the ZSM-5 type zeolite catalyst with, forexample, silica, phosphorus, metal ions or metal oxides, can enhanceselectivity of the methanol conversion reaction for production of lightolefins. Likewise, utilization of dilute methanol feeds or inertdiluents can also tend to increase selectivity of the reaction towardethylene and light olefin production. Notwithstanding the existance ofprocesses suitable for converting methanol to high yields of lightolefins, there is a continuing need to develop additional catalyticprocedures suitable for converting an organic charge comprising methanolto light olefin products with improved ethylene and light olefinselectivity.

Accordingly, it is an object of the present invention to provide animproved process for converting a methanol feed to olefin-containingproducts with high selectivity to production of light olefins.

It is a further object of the present invention to provide such aselective methanol conversion process which can be employed inconjunction with known catalysts and processes for maximizing lightolefin production from methanol.

It is a further object of the present invention to provide such aselective methanol conversion process employing conventional catalysts,readily available reactants and diluents and commercially practicalreaction conditions.

SUMMARY OF THE INVENTION

In accordance with the present invention, a process is provided for theselective conversion of a methanol feed to a hydrocarbon mixture rich inlight olefins. The catalyst employed in such a process comprises atleast some crystalline aluminosilicate zeolite material characterized bya Constraint Index of about 1 to 12 and a silica to alumina molar ratioof at least about 12, e.g. ZSM-5. Methanol conversion over such acatalyst occurs in a reaction zone under conversion conditions in thepresence of aldehyde-containing diluent comprising formaldehyde,acetaldehyde or mixtures thereof. This aldehyde-containing diluent canbe co-fed to the reaction zone in an amount sufficient to increase theselectivity of the conversion reaction to production of C₂ -C₄ olefins.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a methanol feed iscatalytically converted to a hydrocarbon product rich in olefins. Theterm "methanol feed" as used herein designates only the organic materialused as feed, i.e the organic compounds subjected to catalyticconversion to olefins, even though the total charge to the conversionreaction zone may contain additional components such as water. Sincemethanol is miscible with water, the charge to the catalytic reactionzone may actually contain large amounts of water, but only the methanol,and associated organic compounds, constitutes the methanol feed. Thus,all computable quantities referred to methanol feed, such as compositionand weight hourly space velocity (WHSV), for purposes of the presentinvention are to be computed on a substantially anhydrous basis.

Any methanol product comprising at least 60 wt. % of methanol may beused to provide methanol for the methanol feed in this invention.Substantially pure methanol, such as industrial grade anhydrousmethanol, is eminently suitable. Crude methanol, which usually containsfrom 12 to 20 wt. % water, or more dilute solutions, also may be used.

Small amounts of impurities such as higher alcohols or other oxygenatedcompounds in the methanol feed have little effect on the conversionreaction of this invention. The methanol feed may contain minor amountsof methyl ether. When this component is present, it is preferred that itconstitute not more than about 20 wt. % of the total methanol feed. Forpurposes of the present invention, it is contemplated to directlyconvert methanol feed to the hydrocarbon mixture characterized by a highcontent of light olefins. Such amounts of methyl ether as may be formedconcomitantly in the conversion reaction, however, may be recovered andrecycled with fresh methanol feed, and the methyl ether contentcalculated on the total of recycle and fresh feed will not ordinarilyexceed the above-noted 20 wt. %.

In one embodiment of the present invention, the charge to the reactionzone comprises only the methanol feed as hereinbefore described. Inanother preferred embodiment of the present process, selectivity of themethanol conversion reaction for production of light olefins can beincreased by contacting methanol feed with the hereinafter describedzeolite based catalyst in the presence of up to about 20 mols, andpreferably from about 1 to 10 mols of steam per mol of methanol feed.Such steam contact is made in the reaction zone under the methanolconversion conditions hereinafter described. Such steam may be provideddirectly by injecting the requisite amount of water or steam into thereaction zone. Alternatively, steam may be provided totally or in partby water mixed with the methanol feed in a molar ratio of water tomethanol feed of up to about 20:1, preferably from about 1:1 to 10:1.Such water in the charge to the reaction zone, of course, forms steam inthe reaction zone under the conversion conditions of the presentinvention.

The methanol feed as hereinbefore described is catalytically convertedto a light olefin enriched hydrocarbon product by contact with acatalyst comprising a particular type of crystalline aluminosilicatezeolite material which exhibits unusual properties. Although suchzeolites have usually low alumina contents, i.e. high silica to aluminamole ratios, they are very active even when the silica to alumina moleratio exceeds 30. Such activity is surprising, since catalytic activityis generally attributed to framework aluminum atoms and/or cationsassociated with these aluninum atoms. These zeolites retain theircrystallinity for long periods in spite of the presence of steam at hightemperature which induces irreversible collapse of the framework ofother zeolites, e.g. of the X and A type. Furthermore, carbonaceousdeposits, when formed, may be removed by burning at higher than usualtemperatures to restore activity. These zeolites, used as catalysts,generally have low coke-forming activity and therefore are conducive tolong times on stream between regenerations by burning carbonaceousdeposits with oxygen-containing gas such as air.

An important characteristic of the crystal structure of this particularclass of zeolites is that it provides a selective constrained access toand egress from the intracrystalline free space by virtue of having aneffective pore size intermediate between the small pore Linde A and thelarge pore Linde X, i.e. the pore windows of the structure are of abouta size such as would be provided by 10-membered rings of silicon atomsinterconnected by oxygen atoms. It is to be understood, of course, thatthese rings are those formed by the regular disposition of thetetrahedra making up the anionic framework of the crystalline zeolite,the oxygen atoms themselves being bonded to the silicon or aluminumatoms at the centers of the tetrahedra. Briefly, the preferred typezeolites useful in this invention possess, in combination: a silica toalumina mole ratio of at least about 12; and a structure providingconstrained access to the intracrystalline free space.

The silica to alumina mole ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although zeolites with a silica to aluminamole ratio of at least 12 are useful, it is preferred in some instancesto use zeolites having substantially higher silica/alumina ratios, e.g.70 and above or even 1600 and above. In addition, zeolites as otherwisecharacterized herein but which are substantially free of aluminum, thatis zeolites having silica to alumina mole ratios of up to infinity, arefound to be useful and even preferable in some instances. Such "highsilica" or "highly siliceous" zeolites are intended to be includedwithin this description. Also to be included within this definition aresubstantially pure silica analogs of the useful zeolites describedherein, that is to say those zeolites having no measurable amount ofaluminum (silica to alumina mole ratio of infinity) but which otherwiseembody the characteristics disclosed.

Members of this particular class of zeolites, after activation, acquirean intracrystalline sorption capacity for normal hexane which is greaterthan that for water, i.e. they exhibit "hydrophobic" properties. Thishydrophobic character can be used to advantage in some applications.

The zeolites of the particular class useful herein have an effectivepore size such as to freely sorb normal hexane. In addition, theirstructure must provide constrained access to larger molecules. It issometimes possible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of silicon and aluminum atoms,then access by molecules of larger cross-section than normal hexane isexcluded and the zeolite is not of the desired type. Windows of10-membered rings are preferred, although in some instances excessivepuckering of the rings or pore blockage may render these zeolitesineffective. Twelve-membered rings usually do not offer sufficientconstraint to produce the advantageous conversions, although thepuckered 12-ring structure of TMA offretite shows constrained access.Other 12-ring structures may exist which may be operative.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access to molecules oflarger cross-section than normal paraffins, a simple determination ofthe "Constraint Index" as herein defined may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a sample of zeolite at atmospheric pressureaccording to the following procedure. A sample of the zeolite, in theform of pellets or extrudate, is crushed to a particle size about thatof coarse sand and mounted in a glass tube. Prior to testing, thezeolite is treated with a stream of air at 540° C. for at least 15minutes. The zeolite is then flushed with helium and the temperature isadjusted between 290° C. and 510° C. to give an overall conversion ofbetween 10% and 60%. The mixture of hydrocarbons is passed at 1 liquidhourly space velocity (i.e., 1 volume of liquid hydrocarbon per volumeof zeolite per hour) over the zeolite with a helium dilution to give ahelium to (total) hydrocarbon mole ratio of 4:1. After 20 minutes onstream, a sample of the effluent is taken and analyzed, mostconveniently by gas chromatography, to determine the fraction remainingunchanged for each of the two hydrocarbons.

The "Constraint Index" is calculated as follows: ##EQU1##

The Constraint Index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Zeolites suitable for the presentinvention are those having a Constraint Index of about 1 to 12.Constraint Index (CI) values for some typical materials are:

    ______________________________________                                        Zeolite             C.I.                                                      ______________________________________                                        ZSM-5               8.3                                                       ZSM-11              8.7                                                       ZSM-12              2                                                         ZSM-23              9.1                                                       ZSM-35              4.5                                                       ZSM-38              2                                                         ZSM-48              3.4                                                       TMA Offretite       3.7                                                       Clinoptilolite      3.4                                                       Beta                0.6                                                       H-Zeolon (mordenite)                                                                              0.4                                                       REY                 0.4                                                       Amorphous Silica-Alumina                                                                          0.6                                                       Erionite            38                                                        ______________________________________                                    

The above-described Constraint Index is an important and even criticaldefinition of those zeolites which are useful in the instant invention.The very nature of this parameter and the recited technique by which itis determined, however, admit of the possibility that a given zeolitecan be tested under somewhat different conditions and thereby exhibitdifferent Constraint Indices. Constraint Index seems to vary somewhatwith severity of operation (conversion) and the presence or absence ofbinders. Likewise, other variables such as crystal size of the zeolite,the presence of occluded contaminants, etc., may affect the constraintindex. Therefore, it will be appreciated that it may be possible to soselect test conditions as to establish more than one value in the rangeof 1 to 12 for the Constraint Index of a particular zeolite. Such azeolite exhibits the constrained access as herein defined and is to beregarded as having a Constraint Index in the range of 1 to 12. Alsocontemplated herein as having a Constraint Index in the range of 1 to 12and therefore within the scope of the defined class of highly siliceouszeolites are those zeolites which, when tested under two or more sets ofconditions within the above-specified ranges of temperature andconversion, produce a value of the Constraint Index slightly less than1, e.g. 0.9, or somewhat greater than 12, e.g. 14 or 15, with at leastone other value within the range of 1 to 12. Thus, it should beunderstood that the Constraint Index value as used herein is aninclusive rather than a exclusive value. That is, a crystalline zeolitewhen identified by any combination of conditions within the testingdefinition set forth herein as having a Constraint Index in the range of1 to 12 is intended to be included in the instant novel zeolitedefinition whether or not the same identical zeolite, when tested underother of the defined conditions, may give a Constraint Index valueoutside of the range of 1 to 12.

The particular class of zeolites defined herein is exemplified by ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48 and other similarmaterials.

ZSM-5 is described in greater detail in U.S. Pat. Nos. 3,702,886 and Re.29,948. The entire descriptions contained within those patents,particularly the X-ray diffraction pattern of therein disclosed ZSM-5,are incorporated herein by reference.

ZSM-11 is described in U.S. Pat. No. 3,709,979. That description, and inparticular the X-ray diffraction pattern of said ZSM-11, is incorporatedherein by reference.

ZSM-12 is described in U.S. Pat. No. 3,832,449. That description, and inparticular the X-ray diffraction pattern disclosed therein, isincorporated herein by reference.

ZSM-23 is described in U.S. Pat. No. 4,076,842. The entire contentthereof, particularly the specification of the X-ray diffraction patternof the disclosed zeolite, is incorporated herein by reference.

ZSM-35 is described in U.S. Pat. No. 4,016,245. The description of thatzeolite, and particularly the X-ray diffraction pattern thereof, isincorporated herein by reference.

ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859. Thedescription of that zeolite, and particularly the specified X-raydiffraction pattern thereof, is incorporated herein by reference.

ZSM-48 is more particularly described in European patent application No.80 300,463 published Sept. 3, 1980 as Publication No. 0015132, theentire content of which is incorporated herein by reference.

It is to be understood that by incorporating by reference the foregoingpatent documents to describe examples of specific members of thespecified zeolite class with greater particularity, it is intended thatidentification of the therein disclosed crystalline zeolites be resolvedon the basis of their respective X-ray diffraction patterns. Asdiscussed above, the present invention contemplates utilization of suchcatalysts wherein the mole ratio of silica to alumina is essentiallyunbounded. The incorporation of the identified patents should thereforenot be construed as limiting the disclosed crystalline zeolites to thosehaving the specific silica-alumina mole ratios discussed therein, it nowbeing known that such zeolites may be substantially aluminum-free andyet, having the same crystal structure as the disclosed materials, maybe useful or even preferred in some applications. It is the crystalstructure, as identified by the X-ray diffraction "fingerprint", whichestablishes the identity of the specific crystalline zeolite material.

The specific zeolites described, when prepared in the presence oforganic cations, are substantially catalytically inactive, possiblybecause the intra-crystalline free space is occupied by organic cationsfrom the forming solution. They may be activated by heating in an inertatmosphere at 540° C. for one hour, for example, followed by baseexchange with ammonium salts followed by calcination at 540° C. in air.The presence of organic cations in the forming solution may not beabsolutely essential to the formation of this type zeolite; however, thepresence of these cations does appear to favor the formation of thisspecial class of zeolite. More generally, it is desirable to activatethis type catalyst by base exchange with ammonium salts followed bycalcination in air at about 540° C. for from about 15 minutes to about24 hours.

Natural zeolites may sometimes be converted to zeolite structures of theclass herein identified by various activation procedures and othertreatments such as base exchange, steaming, alumina extraction andcalcination, alone or in combinations. Natural minerals which may be sotreated include ferrierite, brewsterite, stilbite, dachiardite,epistilbite, heulandite, and clinoptilolite.

The preferred crystalline zeolites for utilization herein include ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48, with ZSM-5 beingparticularly preferred.

In a preferred aspect of this invention, the zeolites hereof areselected as those providing among other things a crystal frameworkdensity, in the dry hydrogen form, of not less than about 1.6 grams percubic centimeter. It has been found that zeolites which satisfy allthree of the discussed criteria are most desired.

Therefore, the preferred zeolites useful with respect to this inventionare those having a Constraint Index as defined above of about 1 to about12, a silica to alumina mole ratio of at least about 12 and a driedcrystal density of not less than about 1.6 grams per cubic centimeter.The dry density for known structures may be calculated from the numberof silicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g.,on Page 19 of the article Zeolite Structure by W. M. Meier. This paper,the entire contents of which are incorporated herein by reference, isincluded in Proceedings of the Conference on Molecular Sieves, (London,April 1967) published by the Society of Chemical Industry, London, 1968.

When the crystal structure is unknown, the crystal framework density maybe determined by classical pycnometer techniques. For example, it may bedetermined by immersing the dry hydrogen form of the zeolite in anorganic solvent which is not sorbed by the crystal. Or, the crystaldensity may be determined by mercury porosimetry, since mercury willfill the interstices between crystals but will not penetrate theintracrystalline free space.

It is possible that the unusual sustained activity and stability of thisspecial class of zeolites is associated with its high crystal anionicframework density of not less than about 1.6 grams per cubic centimeter.This high density must necessarily be associated with a relatively smallamount of free space within the crystal, which might be expected toresult in more stable structures. This free space, however, is importantas the locus of catalytic activity.

Crystal framework densities of some typical zeolites, including somewhich are not within the purview of this invention, are:

    ______________________________________                                                     Void           Framework                                                      Volume         Density                                           ______________________________________                                        Ferrierite     0.28   cc/cc     1.76 g/cc                                     Mordenite      .28              1.7                                           ZSM-5, -11     .29              1.79                                          ZSM-12         --               1.8                                           ZSM-23         --               2.0                                           Dachiardite    .32              1.72                                          L              .32              1.61                                          Clinoptilolite .34              1.71                                          Laumontite     .34              1.77                                          ZSM-4 (Omega)  .38              1.65                                          Heulandite     .39              1.69                                          P              .41              1.57                                          Offretite      .40              1.55                                          Levynite       .40              1.54                                          Erionite       .35              1.51                                          Gmelinite      .44              1.46                                          Chabazite      .47              1.45                                          A              .5               1.3                                           Y              .48              1.27                                          ______________________________________                                    

When synthesized in the alkali metal form, the zeolite is convenientlyconverted to the hydrogen form, generally by intermediate formation ofthe ammonium form as a result of ammonium ion exchange and calcinationof the ammonium form to yield the hydrogen form. In addition to thehydrogen form, other forms of the zeolite wherein the original alkalimetal has been reduced to less than about 1.5 percent by weight may beused as precursors to the alkaline-earth metal modified zeolites of thepresent invention. Thus, the original alkali metal of the zeolite may bereplaced by ion exchange with other suitable metal cations of Groups Ithrough VIII of the Periodic Table, including, by way of example,nickel, copper, zinc, palladium, calcium or rare earth metals.

In practicing methanol conversion processes using the catalysts of thepresent invention, it may be useful to incorporate the above-describedcrystalline zeolites with a matrix comprising another material resistantto the temperature and other conditions employed in such processes. Suchmatrix materials include synthetic or naturally occurring substances aswell as inorganic materials such as clay, silica and/or metal oxides.The latter may be either naturally occurring or in the form ofgelatinous precipitates or gels including mixtures of silica and metaloxides. Naturally occurring clays which can be composited with thezeolite include those of the montmorillonite and kaolin families, whichfamilies include the sub-bentonites and the kaolins commonly known asDixie, McNamee-Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the zeolites employed herein maybe composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, and silica-titania, as well as ternary compositions,such as silica-alumina-thoria, silica-alumina-zirconia,silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may bein the form of a cogel. The relative proportions of zeolite componentand inorganic oxide gel matrix, on an anhydrous basis, may vary widelywith the zeolite content ranging from between about 1 to about 99percent by weight and more usually in the range of about 5 to about 80percent by weight of the dry composite.

The zeolite composites as described may be further modified if desiredto alter their catalytic properties with respect to their utility inpromoting conversion of methanol to a hydrocarbon product. It is known,for example, to modify such zeolites by incorporating various materialsinto or onto such zeolite composites prior to their use for methanolconversion. For example, Rodewald; U.S. Pat. No. 4,100,219; Issued July11, 1978 discloses zeolite-based methanol conversion catalysts treatedwith a silicone or silane material to incorporate amorphous silica withthe zeolite. Rodewald; U.S. Pat. No. 4,066,714; Issued Jan. 3, 1978discloses zeolite-based methanol conversion catalysts modified byincorporation of metal cations having an ionic radius exceeding 1Angstrom, e.g. cesium and barium. Kaeding; U.S. Pat. No. 4,049,573;Issued Sept. 20, 1977 discloses zeolite-based methanol conversioncatalysts modified by incorporation of oxides of boron or magnesiumand/or phosphorus. Butter; U.S. Pat. No. 3,979,472; Issued Sept. 7, 1976discloses zeolite-based methanol conversion catalysts modified byincorporation of antimony oxide. Kaeding et al.; U.S. Pat. No.3,911,041; Issued Oct. 7, 1975 discloses zeolite-based methanolconversion catalysts modified by incorporation of oxides of phosphorus.All of the foregoing patents are incorporated herein by reference.

The process of the present invention involves utilization of theabove-described catalyst compositions, whether modified or not, topromote the selective conversion of methanol feed to hydrocarbons,particularly light (C₂ -C₄) olefins. The methanol feed as hereinbeforedescribed can be contacted in the vapor phase with the particularcatalyst materials, also hereinbefore described, in a reaction zone andunder reaction conditions suitable for effecting conversion of themethanol feed to olefins. Such conversion conditions include anoperating temperature between about 200° C. and 500° C., preferably 275°C. and 425° C., a pressure between about 5 psia (˜35 kPa) and 500 psia(3447 kPa), preferably about 15 psia (103 l kPa) and 100 psia (689 kPa);and a weight hourly space velocity (WHSV) of the methanol feed ofbetween about 0.05 and 30, preferably 0.1 and 10.

In accordance with the present invention, it has been discovered thatespecially desirable light olefin-selective methanol conversion resultscan be achieved by conducting the conversion reaction under theaforementioned reaction conditions and in the presence of analdehyde-containing diluent comprising formaldehyde, acetaldehyde ormixtures thereof. The preferred aldehyde for use in or as the diluent isformaldehyde (HCHO). The diluent can thus comprise substantially pureformaldehyde gas or can comprise aqueous solutions of formaldehyde.Commercial grades of formaldehyde are quite acceptable including the37%, 44% and 50% aqueous solutions known as formalin. Such formalinsolutions generally also contain up to 15% methanol to inhibitformaldehyde polymerization.

The aldehyde-containing diluent can also comprise acetaldehyde, eitheras the sole aldehyde or in admixture with formaldehyde. Acetaldehyde(CH₃ CHO) is generally available as technical grade 99% acetaldehyde.

Formaldehyde and/or acetaldehyde from any source can be admixed withinert liquid or gaseous materials to form the diluent co-fed to thereaction zone along with the methanol charge. Such inert materialsinclude water, hydrogen, nitrogen, carbon dioxide, C₁ to C₇ hydrocarbonsor flue gas.

The aldehyde-containing diluent may be admixed with the methanol feed orcan be introduced separately into the reaction zone. It is alsocontemplated that aldehyde diluent can be provided at least in part byconverting a fraction of the methanol feed to formaldehyde in thepresence of a dehydrogenation or oxidation catalyst.

Such an aldehyde-containing diluent from whatever source can be fed tothe reaction zone in an amount sufficient to increase the selectivity ofthe conversion reaction to the production of C₂ to C₄ olefins. Generallya molar ratio of aldehyde to methanol of about 0.001:1 to 0.1:1 can beemployed. More preferably the aldehyde to methanol molar ratio variesfrom about 0.005:1 to 0.05:1. The diluent can be co-fed to the reactionzone using a weight hourly space velocity (WHSV) of from about 0.00025to 1.5, preferably from about 0.001 to 0.1, based upon pure aldehyde.

The methanol conversion process described herein may be carried out as abatch-type, semi-continuous or continuous operation utilizing a fixed,fluidized or moving bed catalyst system. A preferred embodiment entailsuse of a catalyst zone wherein the alcohol charge together withaldehyde-containing diluent and optionally with added water is passedconcurrently or countercurrently through a fluidized or moving bed ofparticle-form catalyst. The latter after use may be conducted to aregeneration zone wherein the aged catalyst can be regenerated byappropriate regeneration procedures. After regeneration, the regeneratedcatalyst can be recycled to the conversion zone for further contact withthe methanol and/or ether containing feed.

The product stream in the process of the present invention containssteam and a hydrocarbon mixture of paraffins and olefins, substantiallydevoid of aromatics. This mixture is particularly rich in light olefins,i.e., ethylene, propylene, and butylene. Generally, a major fraction ofthe total olefins is ethylene plus propylene with the ethylene contentof the product exceeding the propylene content. Thus, the predominanthydrocarbon product constitutes valuable petrochemicals. The steam andhydrocarbon products may be separated from one another by methods wellknown in the art. In a preferred embodiment of the invention, theunconverted methanol and/or methyl ether, as well as at least part ofthe water in the product, can be recycled to the reaction zone.

The following examples will serve to illustrate the process of thisinvention without limiting the same.

EXAMPLE I

Methanol feeds containing methanol/water mixtures in combination withvarious types of diluents are catalytically converted to hydrocarbonproducts over a ZSM-5 zeolite having a crystallite size of about 1micron and a silica to alumina molar ratio of about 70:1. The zeolite isadmixed with alumina binder to form a composite comprising 65% zeoliteand 35% binder. In one instance, the zeolite is pretreated by contactingit with steam at 1000° C. for 7 hours.

About 1.5 grams of such a catalyst is placed in a reactor maintained atabout 300° C. Various methanol-containing charges are fed to the reactorat 9.24 ml/hr. In some instances, an aldehyde diluent is alsointroduced. Temperature is adjusted within the 300°-375° C. range toobtain CH₂ conversion (methanol and methyl ether) values of about 50%.For 50% CH₂ conversion, selectivities to ethylene and C₂ -C₄ olefins aredetermined by interpolation. Catalyst type, reactant descriptions, spacevelocities and selectivities to ethylene and C₂ -C₄ olefins are setforth in Table I.

                  TABLE I                                                         ______________________________________                                        Light Olefins from Methanol Over HZSM-5/Al.sub.2 O.sub.3                                                         Total                                                                C.sub.2.sup.=                                                                          C.sub.2.sup.= -C.sub.4.sup.=               Catalyst     WHSV         (Wt %)   (Wt %)                                     ______________________________________                                        A.  HZSM-5       MeOH      3.1  27.7   53.6                                                    H.sub.2 O 5.3                                                B.  HZSM-5       MeOH      3.1  31.1   59.5                                                    H.sub.2 O 5.3                                                                 HCHO      0.03                                               C.  Steamed HZSM-5                                                                             MeOH      1.15 30.0   63.1                                                    H.sub.2 O 1.94                                                                HCHO      0.01                                               D.  HZSM-5       MeOH      3.1  30.2   58.7                                                    H.sub.2 O 5.3                                                                 CH.sub.3 CHO                                                                            0.03                                               ______________________________________                                    

The Table I data indicate that selectivity of the methanol conversionreaction to both ethylene and total C₂ -C₄ olefin production can beimproved by co-feeding formaldehyde or acetaldehyde to the reactionzone.

What is claimed is:
 1. In a process for producing a hydrocarbon mixturecontaining light olefins by contacting a methanol feed with a catalystcomprising a crystalline aluminosilicate zeolite having a ConstraintIndex of about 1 to 12 and a silica to alumina molar ratio of at leastabout 12, said contacting occurring in a reaction zone under methanolconversion reaction conditions, the improvement whichcomprises:co-feeding to said reaction zone along with said methanol feedan aldehyde-containing diluent comprising formaldehyde, acetaldehyde ora mixture thereof, in an amount sufficient to increase the selectivityof the conversion reaction for production of C₂ to C₄ olefins.
 2. Aprocess according to claim 1 wherein the aldehyde component of saiddiluent is formaldehyde.
 3. A process according to claim 1 wherein saidzeolite is selected from ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38and ZSM-48.
 4. A process according to claim 3 wherein the methanolconversion reaction conditions include a temperature of from about 200°C. to 500° C., and a reaction zone pressure from about 5 psia to 500psia.
 5. A process according to claim 3 wherein the weight hourly spacevelocity of the aldehyde in said diluent ranges from about 0.00025 to1.5 and the molar ratio of aldehyde to methanol ranges from about0.001:1 to 0.1:1.
 6. A process according to claim 5 wherein said diluentcomprises mixtures of formaldehyde and water.
 7. A process according toclaim 6 wherein said methanol feed is admixed with water and with saiddiluent such that water is present in an amount of from about 1 mol to10 mols of water per mole of methanol.
 8. A process according to claim1, 2, 3, 4, 5, 6 or 7 wherein said catalyst further comprises a binderfor said zeolite material.